1. Field of the Invention
The present invention relates to the gain equalization for optical fibers transmitting optical signals at multiple wavelengths.
2. Description of Prior Art
A metropolitan area network or wide area network (MAN/WAN) using WDMA/WDM has the potential near-far problem where the optical signals originating From different locations experience different attenuation. Furthermore, fibers and optical amplifiers used in such a network also have nonflat transmission spectra and thus different wavelengths experience different gain and attenuation even though they travel through the same physical path. The non-uniform gain of the amplifiers yields different gain factors for different signals distributed across its spectrum. The problem becomes severe as many optical amplifiers are cascaded together. This effect reduces the system margin and may exceed the dynamic range of the detectors. Gain equalization is thus needed.
Most of the existing studies on gain equalization has been focused on equalizing the nonflat gain spectra of the optical amplifiers. Gain equalization using fiber grating embedded in the Er.sup.3+ fiber amplifier was proposed in [1]. A two-stage amplifier with offset gain peaks was proposed in [2] to dynamically equalize (by means of pump power) the optical signal power among different channels in a WDMA system. This scheme, however, has a very limited equalized bandwidth of approximately 2.5 nm. Gain equalization can also be achieved through controlling the transmission spectra of the optical filters. Using this principle, an equalized 29-channel WDM system spanning 7 nm was demonstrated using a Mach-Zehnder Interferometric filter [3]. Besides not having enough bandwidth, the Mach-Zehnder filter is very difficult to control for proper attenuation. An acousto-optic tunable filter (AOTF) has also been used to equalize gain spectra for a very wide transmission window [4]. Through the injection of multiple RF frequencies with each RF signal transmitting at a different magnitude, an arbitrary transmission characteristic can be established. The resolution of this scheme, however, is limited by the frequency of an AOTF.
Therefore, there is a need for a low cost, wideband, passive, optical device to provide for optical communication over wide area and long haul communication networks.